STUDY ON FLOW BOILING HEAT TRANSFER IN HORIZONTAL-RECTANGULAR-NARROW-FLAT CHANNELS

摘要

This study is intended to examine to how large conventional results on the pressure drop and the heat transfer can be applicable when the channel size is decreased. By using deionized water, single-phase flow and flow boiling heat transfer experiments were performed for a thin rectangular channel of the width W = 3 mm and the height δ = 1.25 mm ~ 0.163 mm. The previous results of authors for the thin rectangular channel of the width W = 10 mm and the height δ=1.1 mm ~ 0.18 mm were also utilized in analyzing results. The channel narrowness effect on the water single-phase flow heat transfer appeared when the hydraulic diameter D_(hy) became smaller than 1.06 mm. D_(hy) = 1.06 mm corresponds to δ = 0.640 mm for the present 3 mm width case and δ = 0.557 mm for the 10 mm width case. When the channel height became narrower than that size, the Nusselt number of the water single-phase flow heat transfer became smaller than that for a conventional size channel in the turbulent flow region. It also became dependent on the Reynolds number even in the laminar flow region. The modification method to incorporate the channel narrowness effect into the heat transfer coefficient correlation for the turbulent flow was proposed. The narrowness effect on the water single-phase flow pressure drop came out at D_(hy) = 0.714 mm i.e. δ = 0.405 mm for W = 3 mm and δ= 0.369 mm for W = 10 mm. When the channel height became narrower than that size, the friction factor in the laminar flow region became smaller than the value of a conventional size channel and transition from the laminar flow to the turbulent flow was delayed. The method to modify the friction factor equation of the laminar flow so as to be applicable to the narrow channel was proposed. In the flow boiling, when boiling started, churn flow appeared even at a low heat flux and a flow pattern was mainly the churn flow or/and the annular flow. The heat transfer coefficient was larger than that of pool boiling. The critical heat flux was higher than the value of Kutateladze correlation for the pool boiling. However, when the flow channel height became extremely narrow in the present experimental range, it became lower than the Kutateladze value. The evaporation heat transfer of a film on the heat transfer surface was dominant. When the modification for the single-phase flow heat transfer was introduced, the Dengler and Addoms correlation for the forced convection evaporation heat transfer for a conventional size channel predicted well present results.